The purpose of the proposed investigation is to provide documentation for the magnitude of changes in the composition of the fat-free body in prepubescent and postpubescent children to enable accurate prediction equations to be developed for estimation of body fat content from various indirect methods. A cross sectional developmental research design will be used to study fat-free body composition of subjects at each of four age groups (8 to 10, 11 through 14, 15 to 17 and 20 to 29 years of age) with male and female subjects from each of 4 age groups and 2 racial backgrounds. Two hundred and ninety-six subjects will comprise the validation sample arranged in a 4 x 2 x 2 factorial plan. The racial groups will include whites and blacks. Components of the fat-free body to be measured include total body water using deuterium dilution, bone mineral content of a cross-section of the radius and ulna (photon absorptiometry), body potassium by whole body 40K spectrometry, body density by underwater weighing, and anthropometry. The study is designed to test the concept that the fat-free body composition of prepubescent children contains a higher water and lower mineral content than that of postpubescent children and adults and that the use of equations for estimation of fatness devised from data on adults leads to an over-estimation of fat content in children. Regression equations between anthropometry and fatness will be crossvalidated on several additional samples of children of various socioeconomic and racial backgrounds. It is anticipated that a generalized approach for the estimation of body fat content can be derived from anthropometric measures in both children and adults using comparable measurement sites.